knuckle duster vs plural form

knuckle duster and plural form both are nouns.

Word	Noun	Adjective	Verb	Adverb
knuckle duster	Yes	No	No	No
plural form	Yes	No	No	No

knuckle duster (noun)	plural form (noun)
(used in the plural) a small metal weapon; worn over the knuckles on the back of the hand	the form of a word that is used to denote more than one